revlog: add an exception hint when processing LFS flags without the extension It would be even better if this was either detected sooner, or the transaction completed (especially since the read/write processors aren't needed for the exchange). But this makes it easier for the user to resolve until that can be figured out.

tests: drop py36 conditionals in test-bad-extension.t Since this is a `>=` test, it's really conditionalizing py27 content, which isn't a thing anymore.

tests: drop py36 conditionals in test-hook.t Since this is a `>=` test, it's really conditionalizing py27 content, which isn't a thing anymore.

tests: drop py36 conditionals in test-http-bad-server.t Since this is a `>=` test, it's really conditionalizing py27 content, which isn't a thing anymore.

configitems: enable changegroup3 by default (unless using infinitepush) The LFS extension requires this, and if it isn't enabled on the client (or the LFS extension isn't loaded), a web client gets a 500 instead of a sensible error message. Now it gets a different (client) error, but maybe it can be handled more gracefully. c0f11347b107 indicates that treemanifest repos have this issue too. 29cfc474c5fd mentions gating this behind `experimental` so that the format could change, but that was 7 years ago and we now have an experimental `changegroup4` as well. We can keep this as a config for the next cycle in case someone runs into an unexpected problem, and then jettison it if the infinitepush bundle name changes are either acceptable as-is or can be created differently.

infinitepush: opt out of changegroup3 unless explicitly configured This is currently a no-op, as changegroup3 is disabled by default. But when it is enabled, it changes the hash names of the bundle files. As I don't use this extension, I have no idea if that's OK or not. So keep the current default behavior until we can get more info from actual users, while allowing them to opt-in for testing purposes.

statichttprepo: unbyteify several IOError messages Builtin errors generally want str messages.

statichttprepo: fix the vfs.join() method to match the base class definition Flagged by PyCharm. The superclass implementation raised NotImplementedError.

mail: unbyteify the SMTPException message This is a subclass of OSError, which wants str instead of bytes.

mail: add a missing argument to properly override starttls I didn't look into when this changed, but it was flagged by PyCharm.